The present invention relates to an air-demand-controlled compressor arrangement, particularly for commercial vehicles, having a motor unit for generating a rotating movement which, by way of an interposed rotational-speed-transmitting transmission unit, drives a compressor unit for generating compressed air from the ambient air. A control unit triggers the generating of compressed air when a compressed-air demand is determined.
Compressor arrangements of the type which is of interest here are predominantly installed in commercial vehicles in order to implement the procurement of compressed air from the on-board compressed-air system. The compressed-air system on-board a commercial vehicle is required, in particular for supplying the braking system, the air suspension, trailers and accessories. For this purpose, the compressor arrangement generates compressed air of up to 12.5 bar.
The product information “High-Performance Compressors” of the firm KNORR-BREMSE Systeme für Nutzfahrzeuge GmbH (Printing No. P-3505DE-01) reveals a compressor unit which is constructed in the form of a piston compressor. A crankshaft rotatingly disposed in a compressor housing converts an input-side rotating movement, by means of a crankshaft drive, into a linear movement of an assigned piston housed in a cylinder. The piston, interacting with a valve device, takes in ambient air and subsequently compresses the latter. Depending on the desired delivery capacity, the compressor unit may be of a single-cylinder or multi-cylinder construction.
Customarily, the driving of the compressor unit takes place by way of the rotating movement generated by the internal-combustion engine of the commercial vehicle. In most application cases, a rotational-speed-transmitting transmission unit is interposed between the internal-combustion engine as the motor unit and the compressor unit. In this case, the transmission unit is constructed as a spur gear transmission with a fixed transmission ratio, which gears up the rotational speed supplied by the motor unit on the input side in order to drive the compressor unit.
Furthermore, in the case of this known compressor arrangement, a control is provided which ensures that the operation of the compressor unit can be started only when a compressed-air demand exists in the compressed-air system of the commercial vehicle. In most cases, the compressed-air demand is determined by way of a pressure sensor connected with the system pressure. When the system pressures falls below a defined threshold pressure, the compressor unit is started in order to again build up a sufficient air pressure. Additional pressure reservoirs are usually used in the compressed-air system, for storing the built-up pressure.
In order to implement such a demand control for starting the compressor unit, the compressor unit can be switched between a delivery phase and a no load phase. In the delivery phase, compressed air is generated from the ambient air and is fed into the compressed-air system. By contrast, the compressor unit runs without any load in the no-load phase so that, although a piston movement takes place, no compressed air reaches the compressed-air system. This compressed air is discharged to the outside. Since, in the no-load phase, as a result of the eliminated load in comparison to the delivery phase, considerably less power is received by the compressor unit, this type of air demand control contributes to the saving of energy.
However, long-term tests have shown that the air-demand-controlled compressor arrangement is often operated with very brief switch-on durations of 5 to 10% in the delivery phase, which is the result of the predominant long-distance hauling operation on a turnpike. In the case of this brief switch-on time, the comparatively long switch-on duration of approximately 90% of the no-load phase becomes quite important so that, because of the still considerable power consumption in the no-load phase, the energy consumption in the no-load operation is, on the whole, higher than in the load operation. This result is intensified by the fact that the compressor unit capacity is often over sized for normal operation in order to generate high pressure in the compressed-air system within a very short time. This applies particularly to the charging of the compressed-air system when the pressure reservoirs are empty, to the operation of lifting axles, etc. Thus, on the whole, the known air demand control for saving energy is still quite unsatisfactory.
Furthermore, in the general state of the art, an air demand control is also known which uses an operable disconnecting coupling between the motor unit and the compressor unit, whereby the compressor unit is stopped when there is no demand for compressed air. However, in comparison to a continuous no-load operation, a compressor unit operated in this manner has fairly high wear as a result of a lack of a lubricating effect during the cold start. Furthermore, also the operable disconnecting coupling required for this air demand control is subject to wear so that, in the case of this alternative solution, on the whole, fairly high maintenance expenditures are required. Another disadvantage is the fact that, because of the complete power flux separation by means of the disconnecting coupling, no possibility exists for driving diverse auxiliary aggregates—such as the steering booster or the hydraulic pump.
It is therefore an object of the present invention to further improve an air-demand-controlled compressor arrangement of the above-mentioned type such that a more effective savings of energy of the compressor arrangement is ensured while the maintenance expenditures are simultaneously kept to a minimum.
The object is achieved based on an air-demand-controlled compressor arrangement, for commercial vehicles, having a motor unit for generating a rotating movement which, by way of an interposed rotational-speed-transmitting transmission unit, drives a compressor unit for generating compressed air from the ambient air. A control unit triggers the generating of compressed air in the case of a compressed-air demand. The transmission unit is constructed in the manner of a transmission whose rotational speed can be varied. The control unit interacts with the transmission unit in order to adapt the rotational speed of the transmission unit corresponding to the compressed-air demand, in which case the compressed-air demand can be determined via a mechanical or electric pressure sensor determining the actual pressure. The control unit connected with the pressure sensor signals a compressed-air demand precisely when the actual pressure falls under a predefined desired pressure, for triggering the transmission unit. Advantageous further developments of the invention are described and claimed herein.
The invention includes the technical teaching that the transmission unit, which is used within the scope of a compressor arrangement, is constructed in the manner of a transmission with a variable rotational speed, and that the control unit interacts with the transmission unit in order to adapt the rotational speed of the transmission unit corresponding to the compressed-air demand.
The advantage of the solution according to the invention particularly is the fact that, in contrast to known measures for saving energy, the high fraction of the energy consumption in the predominant no-load operation of the compressor unit is considerably reduced because the rotational speed of the compressor unit is decreased in this case. The adaptation of the compressor performance to the pressure demand achieved thereby is therefore based on an analogous variation of the rotational speed of the compressor, so that a direct dependence of the rotational speed of the compressor on the rotational speed of the engine and a conventionally interposed transmission with a fixed transmission ratio is eliminated.
Furthermore, in the case of the solution according to the invention, the control unit has an influence on the transmission connected in front of the compressor unit and not directly on the compressor unit, in order to cause a savings of energy. Because the air demand control here does not operate by way of only a switching-on and switching-off of the compressor unit, the wear within the compressor arrangement remains low. Also, the solution according to the invention permits the driving of diverse auxiliary aggregates—such as the brake booster or the hydraulic pump—permanently at the rotational speed minimally required for this purpose.
The transmission unit whose rotational speed can be varied may be constructed as a change-speed gear box which can be shifted by way of the control unit. Tests have shown that a 2-position change-speed gear box is completely sufficient for this purpose. However, the transmission unit whose rotational speed can be varied may also be constructed as a continuously adjustable transmission whose triggering also takes place by way of the control unit. A continuously adjustable transmission does not require high-expenditure automatic operating devices and provides the spectrum of the transmission ratio required for the present application.
The triggering of the transmission unit for varying the rotational speed can take place, for example, by way of a pneumatic signal generated by the control unit. It is also contemplated to implement the triggering by way of an electric signal. In the latter case, the electric signal for triggering the transmission unit should advantageously be generated by a higher-ranking vehicle control in which the control unit for the transmission unit is integrated.
The compressed-air demand is preferably determined by means of an electric pressure sensor determining the ACTUAL pressure, in which case the control unit connected with the pressure sensor signals a compressed-air demand precisely when the ACTUAL pressure falls below a predefined DESIRED pressure, for triggering the transmission unit. However, it is also contemplated to use, as an alternative to an electric pressure sensor, a mechanical pressure sensor which detects the ACTUAL pressure in the compressed-air system, for example, by way of a spring-loaded diaphragm-tappet arrangement, or the like.